1. Field of the Invention
This invention relates to the analysis of a component in a test sample whereby a reactant system produces a detectable response upon contact with the component, and wherein the production of the detectable response ceases after a predetermined time period has elapsed.
2. Discussion of the Prior Art
The fields of diagnostic physics and chemistry have expanded at a phenomenal rate over the past 25 years such that, especially in the medical area, diagnosis of system parameters can be made with incredible facility and speed.
One such area of expansion has been that of medical diagnostics whereby numerous bodily functions can be studied merely by dipping a reagent-laden strip into a sample of bodily fluid, such as urine, and observing a detectable response such as a color appearance or change, or a change in the amount of light reflected from or absorbed by the strip.
Compatible with such "dip-and-read" methods have arisen numerous chemistries for detecting bodily fluid components. Most of these produce a detectable response which is quantitative or at least semi-quantitative. Thus, by measuring the response after a predetermined time, the analyst can obtain, not only a positive indication of the presence of a particular constituent in a bodily fluid, but also an estimate of how much of the constituent is present. Hence, such strips provide the physician with a facile diagnostic tool as well as the ability to gauge the extent of disease or bodily malfunction.
Illustrative of such strips currently in use are products available from the Ames Company Division of Miles Laboratories, Inc. under the trademarks CLINISTIX, MULTISTIX, KETOSTIX, N-MULTISTIX, DIASTIX, PHENISTIX, DEXTROSTIX, and others. Test devices such as these usually comprise one or more carrier matrices, such as absorbent paper, having respectively absorbed on them a particular reactant system which manifests a color change or appearance in the presence of a specific test sample component. Depending on the specific reactant system incorporated with a particular matrix, these devices can detect the presence of glucose, albumin, ketones, bilirubin, occult blood, nitrite, urobilinogen, hydrogen ion concentration (pH) or the like. The specific color appearance and its intensity observable within a specific time range after contacting the strip with the sample is indicative of the presence of a particular component and its concentration in the sample. Some of these test devices and their reactant systems are set forth in U.S. Pat. Nos. 3,123,443 (CLINISTIX); 3,212,855 (KETOSTOX); 3,814,668, 3,164,534 and 2,981,606 (DIASTIX); and 3,092,465, 3,298,789, 3,164,534 and 2,981,606 (DEXTROSTIX).
Typically, diagnostic reagent tests, such as "dip-and-read" reagent strips are accompanied by detailed printed instructions which must be carefully followed to assure accuracy. In the case where the detectable response is a color change, particular care is required. A chart of varying colors for comparison with the strip is provided, and, since in most cases the quantitativeness of the device is dependent upon determining the degree of color formation with respect to time, it is imperative that the color change be compared with the chart within a prescribed time range after dipping into the sample. Waiting periods must be accurately adhered to - too early a reading resulting in too little color formation, and too late a reading resulting in too intense a color formation or even the occurance of an ancillary, intervening color. Hence, if the color formation is compared with the color chart too early or too late, an inaccurate result can be, and often is, obtained.
In an attempt to eliminate the criticality of accurate timing in reading, which is both inconvenient and potentially inaccurate, an extensive research program was engaged in to find a way of precluding the necessity of timing the reading of prior art test devices. Primarily, a way was sought whereby the production of a detectable response would automatically be terminated after a predetermined time, and where the response would remain constant for relatively long storage periods. Thus, color chart comparison could be made at the convenience of the user, or at a time remote from the actual contacting of the device with the sample. For a long time there has been a consensus of opinion in the art of diagnostic reagent chemistry that such test devices would dramatically enhance the state of the art. Yet, to date none has been proposed which would successfully provide the long-awaited solution. None, that is, until the discovery of the present invention.